1. Field of the Invention
The present invention relates to dimensional measurement and, more particularly, to a system for the measurement of gap and mismatch between opposing surfaces.
In the industrial field it is often necessary to measure spaces which exist between two opposing surfaces with the aim, for example, of ensuring that these are indeed approximately equal to predetermined values and to access how these values change with time. The gap which exists between two surfaces is often significant from the point of view of the mechanical operation of the ensemble. The mismatch is closely related to the gap in certain complex configurations where parts are in contact, but more often it is related to aesthetic and aerodynamic considerations. FIG. 1 is a diagram illustrating the gap and mismatch between two opposing parts.
2. Description of the Prior Art
The conventional method for measuring gap and mismatch, particularly in the automobile or aeronautical industries, consists in the use of calipers or gauge blocks. The measurement instrument itself in often associated with a housing including components for storing and processing the collected data, and/or is connected to a computer or printer. These conventional systems have a number of disadvantages.
Firstly, any measurement system involving physical contact runs the risk of damaging the surfaces on which the measurement is made. Besides the problems of creating scratches on the surfaces, the stress exercised on the surfaces by the arms of a set of calipers can cause an increase in the size of the gap between the surfaces and, thus, distort the measurement. The latter problem is particularly serious in the case of measurements performed on thin cantilevered metal panels. Moreover, with these conventional systems it is not possible to satisfy the requirements of resolution and speed of measurement which are becoming more and more severe in the industrial sphere. Finally, and more importantly, the quality of the measurements made with such instruments is highly dependent upon the positioning of the apparatus with respect to the surfaces upon which the measurement is to be made. Thus the repeatability and reliability of the measurements that are obtained is highly dependent on the user.
More recently, non-contact systems, such as, for example, an optical triangulation system, have been proposed for the measurement of gap and mismatch. (An example of a system for measuring gap and mismatch by optical triangulation consists of the product known by the name AFFLEUREDIX, manufactured by the company EDIXIA). The general construction of such a system is illustrated in FIG. 2.
The basic principle underlying the system for measuring gap and mismatch by optical triangulation shown in FIG. 2 is derived from photogrammetry. A laminar beam of laser light is generated by a source (for example a laser diode 5) in association with optical elements. This planar sheet of light is projected onto the surfaces 1,2 on which the measurement is to be performed, in such a way as to illuminate a region including the interface between these surfaces and thus to create a brightness line, if possible perpendicular to the centre of the gap. The brightness line is located within the field of view of a video camera 7 which comprises an array (CCD type) image sensor. The array sensor of the camera is illuminated by the rays coming from the brightness line.
By virtue of a preliminary calibration step the internal geometry of the camera 7 is known and it is thus possible to associate to each pixel of the sensor a straight line in space. Thus, a straight line in space can be associated with each image point on the brightness line. The intersection between this straight line and one of the rays making up the incident light plane creates a triangle located in a plane called the "epipolar plane". The preliminary calibration also enables the separation between the camera 7 and the light source 5 (the baseline) to be determined, as well as the angle between this baseline and each point on the brightness line. For each point on the brightness line, it is thus possible to resolve the corresponding triangle located in the epipolar plane in order to determine the position of this point in three dimensions.
When the three-dimensional positions of the points on the brightness line have been determined, the positions of the edges of the two opposing surfaces are found and then the separation (the gap and mismatch) between these edges is found. The calculation of mismatch involves the determination of the location of the major surface of each of the opposing parts and the calculation of the height difference between these major surfaces.
The known optical triangulation systems enable rapid measurements of gap and mismatch to be made without risk of damaging the surfaces. However, the quality of the measurement that is made still depends upon the orientation of the measurement apparatus. This problem will now be explained with reference to FIG. 3.
FIG. 3 illustrates the case of a projected brightness line which is not perpendicular to the centre of the gap. In such a case, the measured value, J*, does not represent the true gap which exists between the surfaces 1 and 2. The true value of gap, J, is equal to J*cos .theta., where .theta. represents the rotation of the measurement apparatus with respect to the orientation which would have produced a brightness line perpendicular to the centre of the gap. The same problem arises in relation to mismatch, when the apparatus is rotated in another direction.
The repeatability of the measurements made with the known systems is also influenced by the choice of the reference points and lines between which the spaces are measured, especially when curved (i.e; convexedly-curved) surfaces are being characterised.
Any algorithm used for calculating gap and mismatch must include a definition of the reference points and lines (or planes) between which the spaces will be measured. The quality and the repeatability of the gap and mismatch measurements depends upon a judicious choice of the definitions of those reference points and lines. The definitions must correspond to locations which are stable between different samples of the parts being tested. They must also facilitate the establishment of a geometrical construction which enables a measurement of gap or mismatch to be obtained which corresponds to the physical dimension which is normally understood by this expression.
The importance of this choice of reference points, lines and planes is particularly great given that the current trend in the design of bodywork tends towards shapes which are more and more curved, more closely resembling non-standard surfaces than combinations of planes and cylinders. The definitions of the reference parameters used in the known systems are not well-adapted to the characterisation of parts having this type of shape.
Here, the expression "non-standard surface" means any geometrical surface which cannot be strictly described by an equation.
The U.S. patent U.S. Pat. No. 5,416,590 describes an apparatus for measuring gap and mismatch between two opposing parts, using optical triangulation principles in order to determine the three-dimensional positions of points situated on two brightness lines created by two converging planes of light. Each of the two brightness lines illuminates the interface between the two parts and preferably extends perpendicularly to the centre of the gap (this orientation representing the ideal case). The measurements made by this apparatus are claimed to be reliable even in the case where the apparatus is rotated by 10.degree. with respect to the ideal orientation. The calculations of gap and mismatch include a step of defining reference lines and planes for each of the opposing parts. For each part, these reference parameters are calculated from position data relating to the two brightness lines (more particularly, based on data relating to the portions of the two lines which are located on this part). The gap and mismatch are then calculated by an analyse of the interval or spacing between the reference parameters determined for each part.
Because the light planes used in the system of U.S. Pat. No. 5,416,590 converge, the separation between the two brightness lines depends upon the distance between the measurement apparatus and the surfaces upon which the measurement is being made. This imposes constraints upon the user in terms of the positioning of the apparatus, because he must ensure that the two brightness lines are located within the field of view of the camera and that they are spaced apart by a distance sufficient to enable them to be resolved by the camera. This problem is solved in U.S. Pat. No. 5,416,590 by obliging the user to place the measuring instrument right up against the parts to be measured. However, this solution involves a risk of damaging the parts. Moreover, the user cannot see the precise spot where the measurement will be made, which makes it more difficult to position the apparatus at the correct location.
The fact that the light planes used in the U.S. Pat. No. 5,416,590 system converge means that each of the planes is incident on the parts at an oblique angle, typically 40 to 45.degree.. The images generated by the intersection of the light planes with the parts thus does not correspond to the geometric shape of the parts but to a distorted version thereof caused by perspective.
Further, in the system of U.S. Pat. No. 5,416,590, the image data is binarized before the calculation of the three-dimensional positions of the reference points. This procedure eliminates the possibility of performing a "subpixel" interpolation of the position of the brightness line within the image. This leads to a loss of resolution. In addition, the calculation of gap and mismatch according to this known system involves the modelling of each part by a plane determined using the least squares method. This method does not enable a reliable result (or, even, any result at all) to be obtained in the case of measurements made on parts having curved surfaces.